In preheat fluorescent lamp circuits, a glow discharge starter is connected in shunt with the discharge path through the lamp and in series with the lamp electrodes. The glow discharge starter contains a pair of electrodes, at least one of which comprises a heat-deformable bimetallic element, within a glass envelope containing an ionizable medium. When heated as a result of a glow discharge within the starter, the bimetallic element bends towards the other electrode.
At starting, the open circuit voltage of the ballast exceeds the breakdown voltage of the starter and produces a glow discharge therein which heats the bimetallic element and causes it to engage the other electrode in the starter. The closure of the switch effectively short circuits the lamp electrodes in series across the ballast, thereupon the increased current flow through the electrodes raises them rapidly to an electron-emitting temperature. Simultaneously, the glow discharge in the starter is extinguished and the bimetallic element begins to cool and withdraw from the contacted electrode. When contact is broken, a voltage pulse induced by the induction of the ballast, appears across the opposed electrodes of the lamp thereby initiating an arc discharge within the lamp. If lamp ignition does not occur after the first voltage pulse, the glow discharge sequence is repeated until lamp ignition occurs. After the main voltage through the lamp has started, the voltage drop thereacross is less than the breakdown voltage of the starter so that the starter does not develop a glow discharge and its electrodes remain disengaged.
Glow discharge starters are generally designed for operation on either ac or dc circuits. Typically, a starter intended for use on only dc circuits requires the electrode which is connected to the positive polarity to have a surface area at least 10 times that of the other electrode.
A typical glow discharge starter intended for use on only ac current applications may encounter problems if operated on dc. More specifically, in a typical ac glow discharge starter containing a single bimetallic electrode, if the post of the counter electrode is connected to the positive polarity while the bimetallic electrode is connected to the negative polarity, a sustained arc may be established between the electrodes. This sustained arc, if not immediately extinguished, may damage the starter electrodes and cause failure of the glow discharge starter.
In the above example, this problem can be prevented if it were possible to insure that the positive polarity is always connected to the bimetallic electrode and the negative polarity is connected to the counter electrode. Since many glow discharge starters are often enclosed within a lamp base or a container and generally do not contain polarity markings, it is difficult to insure the proper polarity to the electrodes.
Lamp ballasts intended for use with emergency lighting systems often operate lamps on ac current during normal operation. However, immediately following a power failure the lamps may briefly encounter a dc voltage before the ballast of emergency lighting system switches to a high frequency ac voltage. If the electrodes of an ac starter happen to be connected to the wrong polarity during this brief encounter with the dc voltage, failure of the starter as described above may occur.
To overcome the difficulties mentioned above, the present invention proposes the incorporation of a second bimetallic element to act as a safety mechanism. In the event that the starter encounters a dc voltage of the wrong polarity, a portion of the second bimetallic element contacts the first bimetallic element so as to extinguish the arc discharge and prevent electrode damage.
The use of a second bimetallic elements in a glow discharge starter is well known. For example, U.S. Pat. No. 2,376,669, which issued to DeToro on May 22, 1945, described a glow discharge starter containing a pair of bimetallic strips 6 and 9.